You're right. But assumedly it's just as hard to become an enlisted member of Starfleet than it is an officer. When you have hundreds of worlds and trillions of citizens, you're never going to have a shortage of potential applicants for your fleet. Unless you're fielding millions of ships, there will never be a shortage. Even if only rare societal oddballs found the idea of serving appealing? Well 1% of a trillion is still 10 billion. The point is, that for every one person we see on screen, there have to be thousands of potential replacements, each just as qualified and driven.
And that's also why they likely wouldn't mind asking compromised people to resign. After all, getting in in the first place is essentially just an act of extreme luck. Everyone there knows they beat a bunch of folks just as qualified as themselves to get their seat. If you view your position as largely due to chance, then if fate hands you a curve ball and also takes that away from you due to chance? Well that seems reasonable.